Power supply units of various kinds are known. In each unit, sensors may be provided for measuring currents and/or voltages to allow feedback control of the unit and achieved desired supply conditions. One type of power supply unit is an uninterruptible power supply (UPS) for conditioning and safeguarding electric supplies to sensitive or critical equipment. A common use for UPS systems is in data centers. To increase capacity and security of supply, UPS units can be connected in parallel, so that a current supplied to a load is the sum of currents supplied by a number of UPS units. In order for the UPS units to perform satisfactorily in parallel operation, accurate measurement of current and voltage is required, so that each measures the same value for an equal voltage.
In order for the measurements to be calibrated accurately, conventional units require manual calibration procedures, for example using a computer and multimeter. Automated calibration procedures would be advantageous. However, although it is relatively easy to provide automatic calibration of voltage measurements between parallel-connected power supplies, procedures for automatic calibration of current measurements are not known.